This invention relates to centrifugal separators of the self-powered kind for separating particulate contaminants from a liquid, such as a vehicle engine lubricant, within a containment rotor to which contaminated liquid is supplied at elevated pressure, and particularly, but not exclusively, relates to low-cost disposable rotors for use with passenger automobile engines.
Self-powered centrifugal separators are well known for separating fluids of different densities or for separating particulate matter from liquids and have long been used in lubrication systems for engines and analogous items of vehicles. Such devices are described in, for example, GB 735658, GB 757538, GB 2160796, or GB 2283694.
The common principle of operation is that a housing contains a rotor which is supported therein to spin at high speed about a substantially vertical axis. The rotor comprises a container to which contaminated liquid lubricant is supplied at elevated pressure along the axis of rotation at one end of the rotor and is ejected from tangentially directed reaction jet nozzles at the other end of the rotor into the housing from which it drains to the engine sump. The energy lost by the ejected liquid effects rotation of the rotor about the axis at a speed, in excess of 8,000 rpm, fast enough for the liquid circulating in, and passing through, the rotor to deposit solid contaminants on radially outward surfaces. For efficient separation, and to ensure that separated contaminants do not interfere with the reaction jet nozzles, the rotor container is provided with a radially inwardly extending partition wall that effectively divides the rotor into a separation chamber, in which the solids collect, and an outflow chamber, to which the cleaned liquid passes by way of a transfer aperture sited near the rotation axis. It is common in modern designs, such as, EP 0193000 and GB 2283694, for this partition wall to extend both radially and axially as what is sometimes referred to as a separation cone, which better holds solids and liquid-containing sludge within the separation chamber if the rotation axis is tilted from the vertical.
There are several criteria associated with successful operation. Lubricant supplied to the rotor has to be available at a significant pressure if the energy lost by its passage through the reaction jet nozzles is to be sufficient to rotate the rotor fast enough to effect centrifugal separation of said contaminated particles. Also, of course, the lubricant passed through the centrifugal separator loses substantially all of its energy in effecting rotation by jet reaction, that is, it is returned directly to the sump and by-passes the normal lubrication utilisation circuits of the engine, so that the centrifugal separator operates in a so-called lubricant by-pass mode. Consequently, there is normally incorporated in the lubricant supply system a pressure responsive valve which inhibits the flow to the centrifugal separator when the supply pressure is below a predetermined level at which the engine might be starved of lubricant if any were diverted and at which the rotor would not operate efficiently even if supplied.
Such a centrifugal separator is normally associated with a conventional full-flow, or through-flow, filter (although the small particle separation capabilities of the centrifugal separator enables a rather coarser mesh filter to be used than otherwise) and it is often arranged that both filter and separator are mounted on a specially designed interface which includes supply ducts and pressure responsive flow control valves, for example as shown in GB 2160449 and GB 2160796.
Diesel engines are particularly well suited to this form of lubricant cleaning, because of problems of small, light particles in the lubricant that result from combustion products and the generally longer intervals between servicing operation than has been normal with gasoline engines. Thus the combination of a coarse mesh, full-flow filter and centrifugal separator is particularly suited to operation of, and has been widely adopted with, commercial vehicles to maximise intervals between servicing operations.
More recently, it has become popular to include similar diesel engines in small passenger vehicles and, irrespective of engine type, for there to be longer intervals between servicing at which lubricant and/or filter elements would be changed, it being conventional now in relation to passenger vehicles that contaminated components of such filter elements are disposable rather than cleanable by the servicing mechanic, who is frequently the vehicle owner.
Thus it follows that in adapting the concept of supplementing a full-flow filter with a centrifugal separator to such small passenger vehicles, the rotor, when it does eventually fill with contaminant, should be disposable and replaceable with a new one rather than cleaned, and notwithstanding the increased interval between replacements, the disposable rotor must be a low cost item. To this end low cost disposable rotor designs exist, for example EP 0193000, and GB 2283694 which rely upon the rotor being formed as a canister from pressed sheet materials.
However, it is found that centrifugal separators, particularly those of such a pressed sheet construction, exhibit characteristics peripheral to their separation functionality which may militate against ready acceptance within the small passenger vehicle environment.
In such passenger vehicles, although the passenger compartment is much closer to the engine and its ancillary components, there is required a much lower noise level than for commercial vehicles. Whereas any additional noise level caused by a spinning centrifugal separator rotor may be considered minimal in terms of passenger perception whilst the engine is running, it is found that an increased noise level continuing after the engine is stopped is particularly objectionable.
When the engine is stopped the centrifuge rotor, which may be spinning up to 10,000 rpm, continues to rotate for a considerable deceleration or wind-down period that may be in the range of 30-60 seconds or even up to 90 seconds, depending upon the supply pressure and temperature of lubricant passing therethrough, and during that wind-down period the noise level may increase as the rotor empties of lubricant and exhibits bearing contact and out-of-balance vibration as the speed falls and the bulk of the liquid moves about within the partially filled container.
The centrifuge rotor is usually mounted for rotation by bearings comprising plain, parallel bushes carried at each end of the rotor and surrounding fixed, vertically extending axle means to form journal bearings.
The bushes are a clearance fit on the axle means to permit unimpeded rotation and the gap between each bush and axle means is exposed to the lubricant supplied to the rotor such that some lubricant escapes along the gap and with the rotation creates a hydrodynamic film that provides easy rotation and significant radial stiffness. Furthermore, it is known to make use of the forces exerted axially on the bushes and rotor, due to exposure of the ends of the bushes to fluid pressure, to counter the effect of gravity pulling the rotor downwardly along the axle means by having the upper bush of smaller diameter than the lower bush whereby the lubricant pressure acts on different areas to lift the rotating rotor. Each of said bushes may therefore also comprise a radially extending flange to bear against an axially facing surface of the housing, as a pressure lift thrust bearing at the upper part of the housing to limit lifting of the rotor and (particularly) as a weight thrust bearing at the lower part of the housing to support its weight in the absence of lubricant pressure.
It will be appreciated that such weight thrust bearing is employed each time that the lubricant supply is stopped and the weight of the rotor is not countered by lubricant supply pressure. Furthermore, it will be appreciated that the supply of lubricant to the journal bearing gaps also effectively ceases with lubricant supply so that as the rotor winds down, the radial bearing stiffness ceases, permitting the rotor to vibrate on the bearings, both journal and thrust, with the transmission of noise by way of the housing to the engine.
The vibration may be exacerbated as the balance of the rotor is affected by way of lubricant draining therefrom and not being replenished.
To the extent that wind-down noise results from such draining of the rotor, co-pending application number GB9511812.1 describes a flow check valve to retain lubricant within the centrifuge rotor.
However it does not address the problem of wind-down noise resulting directly from the reduction in bearing stiffness as the and hydrodynamic film between bushes and axles means disrupts when the lubricant pressure subsides.